1. Field of the Invention
The present invention relates to a brushless motor driven by a DC power source such as a battery, and more particularly to a brushless motor that drives a device mounted on an automotive vehicle such as a fuel pump.
2. Description of Related Art
A brushless motor is composed of a magnet rotor and a stator having a multi-phase armature winding and is driven by sequentially switching power supply to each phase winding of the armature winding. The power supply timing is controlled based on a rotational position of the magnet rotor detected by a rotor position detector such as a hall-effect element or by induction voltage of the armature winding. A rotational speed of the brushless motor (referred to as a motor speed) is dependent on a power source voltage, i.e., the motor speed decreases as the power source voltage decreases. It is therefore desirable to keep the power source voltage constant to drive the brushless motor at a constant speed.
However, when the power source is an on-board battery, its voltage varies according to various conditions. For example, the battery voltage decreases when the battery is exhausted, and especially it considerably drops when a stator motor is operated. If the motor speed decreases due to the battery voltage drop, a required power cannot be obtained from the motor. To cope with this problem, JP-B2-2560855 proposes to use a voltage stabilizer or a pulse-width modulation control circuit. Such a device, however, is costly and does not meet a cost level required.